As the sun is one of the most sustainable and reliable future sources of energy, photovoltaic (PV) devices are highlighted in both research and industrial areas. A photodiode in the PV devices creates a pair of charge carriers when an incident photon has an energy larger than the energy band gap (Eg) of the semiconducting material of the photodiode. The incident photons with energy higher than Eg contribute to the photo-current and photons with energy lower than Eg cannot. So the conversion efficiency of this process is maximized when all the incident photons carry the exact energy to create the charge carriers.
However, sunlight is not monochromatic but spans a wide range of wavelengths from ultraviolet to infrared. As such, the total energy carried by photons from the sun is not efficiently converted to electricity. To overcome this limitation, thermophotovoltaic (TPV) devices are being considered. The TPV devices consist of a thermal emitter and a photodiode. If the thermal emitter can absorb all incoming photons without discrimination and re-emit photons within a narrow range of energy, optimized for the Eg of the photodiode, in principle, all energy carried by the incident photons can contribute for electricity generation, which results in enhanced energy conversion efficiency.
The thermal radiation properties of the emitter must match the conversion characteristics of the photodiode to optimize energy conversion efficiency. For thermal emission in a narrow range, periodic microstructures including photonic crystals can be considered. In addition to the tuning of the energy of the re-emitted photons, the polarization of the photons is also important in the conversion efficiency of the photodiode as a photon polarized parallel to its plane-of-incidence can be absorbed without reflection loss at the Brewster's angle. The reflection of incoming photons at the surface of the photodiode is significant because of the high refractive index of most semiconductor materials and can result in major efficiency loss unless a high-cost antireflection layer is applied.
Generally, thermal radiation from a thermal source is considered unpolarized or weakly-polarized, which means the two polarizations of thermal radiation are equally distributed. However, a class of micro-structures, called polarized thermal emitters (PTEs), can emit polarized thermal radiation. Because the PTEs preferentially emit polarized photons via their structural anisotropy and not by filtering, the energy loss that always accompanies filtering is avoided. This is a clear and significant advantage for TPV devices.
A good PTE for TPV should show high radiation power only within a selected range of wavelengths and high extinction ratio, defined by P1/P2, where P1 and P2 are the radiation powers for two orthogonal polarizations.
Embodiments of the present invention provide such a PTE for use in a TPV device and methods of manufacturing same. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.